[Part of ENG-843](https://linear.app/comm/issue/ENG-843)
Adds a new `'text'` variant to `Button` and uses it instead of plain html elements in:
- settings screen
- retry send button
- user list in thread composer
to keep the web app more consistent.
There's a slight change in buttons for thread composer (the corners are rounded a bit, and there's a slight animation for hover background change):
{F195909}